Hydrogen has been identified as a flexible fuel form that:
1. permits storage of energy over a wide range of time periods (from hours to years);
2. enables efficient energy transport over long distances within available distribution networks; PA1 3. offers chemical and physical characteristics which propose several distinct applications:
as a chemical feedstock, PA2 as a fuel for electrochemical fuel cell systems in decentralized power generating stations, and PA2 as a fuel supplement to natural gas.
Most of the hydrogen is currently produced through steam reforming of natural gas. Natural gas is a depleting, finite resource as are all of the other fossil fuel sources and is becoming increasingly expensive. New hydrogen production technologies are in the process of development such as coal gasification, water electrolysis or thermochemical water splitting. While coal gasification appears to be the most likely alternative for large scale production of hydrogen, the other technologies appear more competitive for supplying small scale uses (i.e. less than 1 million cubic feet a day).
Thermocemical processes are being explored since thermal energy available from thermonuclear reactors and/or from solar collectors can be fixed as hydrogen, a storable fuel. The decomposition of water by thermochemical means proceeds according to the reaction: EQU H.sub.2 O(L).dbd.H.sub.2 (G)+1/2O.sub.2 (G) (1)
An analysis of the thermodynamics of the cycle requires that energy and entropy be supplied in the cycle. The main feature of this reaction is its highly endothermic nature requiring an input of 40,000 kj/kg-mol. Therefore, the reaction must be practiced as close as possible to ideal conditions in order to be practical. Raising the temperature of the reaction will increase the change in positive entropy and also increases the rate of change of the extent of reaction with temperature.
In a number of thermochemical cycles under active investigation, the oxygen release step is the thermal decomposition of sulfuric acid: ##STR1##
This reaction is highly endothermic and the temperature change required to reduce the Gibb's function to zero is estimated to be about 510 K. Since the temperature difference occurring between the boiling point of sulfuric acid (350.degree. C.) and the temperature available from a high-temperature nuclear reactor (HTR) (.about.850.degree. C.) is about 500.degree. C. and the match of the changes in Gibb's function and entropy for the reaction (2), the reaction is often used as the oxygen release step in thermochemical cycles. The aqueous sulfuric acid must be concentrated and vaporized at 330.degree. C. to 350.degree. C. before the high temperature cracking or decomposition step at 830.degree. C. Maximum corrosion occurs at the vaporization point where sulfuric acid liquid vaporizes to sulfuric acid vapor.
Conventional tubular heat exchangers are thermally inefficient and require the use of expensive forged alloy tubes. It has been suggested that a direct fluid contact heat exchange with H.sub.2 SO.sub.4 would have energy saving benefits over conventional heat exchange in the vaporization of H.sub.2 SO.sub.4 at 330.degree. C. Direct contact heat exchange occurs when two immiscible fluids at different temperatures are mixed. When one of the two fluids undergoes a change of phase, extremely high heat transfer rates result.
Liquids used for heat exchange in direct contact with H.sub.2 SO.sub.4 must meet the following criteria:
1. Liquids must be chemically stable to concentrated sulfuric acid in the temperature range of 300.degree. to 400.degree. C. for the required service periods.
2. Liquids must have very low miscibility with sulfuric acid at use temperatures.
3. Liquids must have low vapor pressure at use temperature to prevent loss by vaporization.
It is also desirable that they should be liquid at room temperature and not be high in cost.
Since the contact is being made with hot concentrated sulfuric acid, most organics are not suitable because of reactivity with the acid. Because of their inertness, fluorine substituted organic materials are the most logical candidates. Under very strong acidic conditions, aliphatic fluorocarbons should be stable because of the very high heats of formation of the carbon-fluorine bond.